This short digital book covers the following essential concepts of Java: Classes, class hierarchies, interfaces, exceptions, and threads. In the process, the notions of applets, object references, polymorphism, overloading, overriding, static members, public and private members, constructors, the Object class, and some key differences between C++ and Java are also explained, to varying degree.This ebook is designed for readers with a background in C wishing to get a quick grasp of the main concepts in Java. To facilitate this, one example is used as far as possible (various shapes: circle, rectangle, etc). While this example has little practical value, it is a good one for explaining the various features in Java to a wide audience. Everyone knows what circles and rectangles are, and can visualize their attributes. The examples in this booklet are designed for readability, not off-the-shelf runnability. Indeed, many examples that build on earlier ones use "..." for the unchanged portions. This booklet is certainly not a comprehensive book on Java. The author has taught a Java course from this material a number of times at the University of California, Berkeley, extension.